Double shielded superconducting field winding

ABSTRACT

In a cryogenic electrical machine having a superconducting field winding mounted on a field structure and an armature mounted on an armature structure, the armature and the field structure carried one by the rotor, the other by the stator of the machine and an inner thermal radiation shield surrounding, mounted with and spaced from the superconducting field winding, an electricalthermal-mechanical outer shield mounted with and spaced from the superconducting field winding and located between the superconducting field winding and the armature, the outer shield being at least co-extensive axially with the superconducting field winding and circumferentially continuous between the superconducting field winding and the armature and disposed in an ambient temperature region of the machine.

United States Patent 1 1 1 1 3,764,835

Luck et al. Oct. 9, 1973 DOUBLE SHIELDED SUPERCONDUCTING PrimaryExaminerD. F. Duggan FIELD WINDING Assignee: Massachusetts Institute ofTechnology, Qambridge, Mass.

Attorney.loseph S. Iandiorio et al.

[5 7] ABSTRACT In a cryogenic electrical machine having asuperconducting field winding mounted on a field structure and [22]Filed: May 1972 an armature mounted on an armature structure, the 21APPL 25 02 armature and the field structure carried one by the rotor,the other by the stator of the machine and an inner thermal radiationshield surrounding, mounted [52] US. Cl. 310/52, 310/ with and Spaced fthe superconducting fi i d [51] Int. Cl. H02k 9/00 ing anelectricauhermabmechanical Outer shield [58] Field of Search 310/10, 40,52 mounted with and spaced from the Superconducting field winding andlocated between the superconduc- [56] References C'ted ting fieldwinding and the armature, the outer shield UNlTED STATES PATENTS beingat least co-extensive axially with the supercon- 3,679,920 7/1972 MacNabet al. 310/10 ducting field Winding and circumfer'amially Continuous3,242,418 3/1966 Mela et a1. 310/10 UX between the superconducting fieldwinding and the ar- 3,368,087 2/1968 Madsen 310/10 mature and disposedin an ambient temperature re- 3,648,082 3/1972 MacNab et al. 310/10 gionf the machine, 3,517,231 6/1970 Massar 310/52 2 Claims, 4 DrawingFigures I I I 44 34 I 1 1 1 45 I 48 32 35 M f W W/ 1 1 1 W 1 It \46"PATENTEDBBT 9W5 5764.835

' SHEET 20F 2v I FIG. 3.

F wa

FIG. 4.

DOUBLE SHIELDED SUPERCONDUCTING FIELD WINDING FIELD OF INVENTION Thisinvention relates to an electrical-thermalmechanical shield for thesuperconducting regions of a cryogenic electrical machine.

BACKGROUND OF INVENTION In a cryogenic electrical machine having asuperconducting field winding the field winding is typically surroundedby a thermal shield to insulate the supercooled region of the fieldwindin. Although the mechanical support for the thermal shield and theshield itself are relatively delicate, i.e., they are thin in order tominimize heat conduction, they nevertheless have sufficient strength towithstand the mechanical forces generated by the eddy currents whichflow in the thermal shield under steady state conditions. However underfault conditions the mechanical forces may reach seven or even ten timesthe normal forces and the heat caused by the increased eddy currents maybe excessive to the extent that the thermal shield is unable to copewith them and may be destroyed. In conventional machines these eddycurrents flow in the surface of the field structure which is composed ofheavy steel and is able to accommodate the high forces and heat, but incryogenic machines that steel is not required or desirable. increasingthe strength and thickness of the thermal shield and its mechanicalsupports increases its thermal conductance to the warm shaft and thesupercooled regions and interferes with its essential function as aninsulator for the supercooled region of the field windings. In additionto proper thermal and mechanical properties the shield must also haveproper conductivity to give the shield an electrical time constant shortenough for good damping of the rotor yet long enough to shield thesupercooled region from transient, zero sequence and negative sequencemagnetic fields. These qualities are explained in greater detail in thethesis submitted in partial fulfillment of the requirements for thedegree of Doctor of Philosophy at the Massachusetts Institute ofTechnology by co-inventor David Lee Luck, Electromechanical and ThermalEffects of Faults Upon Superconducting Generators, June 1971, availableat the library of the Massachusetts Institute of Technology and a copyof which has been enclosed herewith for deposit in the US. Patent OfficeScientific Library SUMMARY OF INVENTION It is therefore an object ofthis invention to provide a shielding device for the supercooled regionof a cryogenic electrical machine having sufficient electricalconductivity to permit an electrical time constant short enough for gooddamping but long enough to shield the low temperature region fromtransient zero sequence and negative sequence magnetic fields, andsufficient mechanical strength to withstand forces generated under faultconditions, while also having sufficient mass, thermal capacity andthermal conductivity to absorb the heat generated during a faultcondition without interfering with the function of thermally insulatingthe supercooled region.

It is a further object of this invention to provide such a devicecapable of having its thermal capacity significantly increased withoutsignificantly increasing the time constant. v

The invention results from the discovery that the combination of theproper thermal and mechanical capacity to withstand fault conditions andthe required electrical characteristics to provide good damping andshielding of transient, zero, and negative sequence magnetic fields canbe realized in one shield and the further discovery that such a shieldcan be separate and distinct from the inner thermal shield whose desiredand good insulative properties can thereby be preserved indifferent toits lack of high thermal capacity, electrical resistivity and mechanicalstrength.

. The invention is utilized in a cryogenic electrical machine having asuperconducting field winding mounted on the field structure and anarmature mounted on an armature structure. The field winding andarmature are carried one by the rotor and the other by the stator of theelectrical machine. The machine also includes an inner thermal shieldsurrounding, mounted with, and spaced from the superconducting fieldwinding. The invention features an electrical-thermal-mechanical outershield mounted with and spaced from the superconducting field windingand located between the superconducting field winding and the armature.The outer shield is at least co-extensive axially with thesuperconducting field winding and circumferentially is continuous withthe superconducting field winding and the armature and is disposed in anambient temperature region of the machine.

DISCLOSURE OF PREFERRED EMBODIMENT Other objects, features andembodiments will occur from the following description of a preferredembodiment and the accompanying drawings in which:

P16. 1 is a schematic cross-sectional view taken through the centralaxis of a cryogenic electrical machine utilizing theelectrical-thermal-mechanical shield according to this invention.

FIG. 2 is a portion of a cross-sectional view taken along lines 22 ofFIG. 1.

FIG. 3 is a view similar to FIG. 1 in which the superconducting fieldwinding is stationary and mounted on an external annular stator whichsurrounds the armature on an internal rotor.

FIG. 4, is a partial cross-sectional view taken along lines 4-4 of FIG.2.

There is shown in FIG. 1 a cryogenic electrical machine 10 having arotor 12 and stator 14; stator 14 includes an armature l6 and anannature structure 18, including laminated iron magnetic shield 20. Theentire stator 14 is at ambient temperature. Rotor 12 includes a centralvacuum cavity 22 formed in rotatable shaft 24-, superconducting fieldwinding 26 is included in a field structure 25 which includes fieldwinding support 28 interconnected, by means of thermal distance pieces30, 32 and 34, 36, with the ends 38 and 40, respectively, of shaft 24.The superconducting field winding 26 is typically cooled to 4.2 K bymeans of a liquid helium cryogenic cooling system, which is not a partof this invention and is omitted for clarity. The superconducting fieldwinding 26 is disposed in vacuum chamber 42 enclosed by inner thermalshield 44 having end pieces 46 and 48, which are interconnected withthermal distance pieces 30, 32, 34, 36. Typically field winding support28 and thermal distance pieces 30, 32, 34, 36 may all be integral partsof shaft 23. Thermal distance pieces 30, 32, 3d and 36 function tothermally shield ambient temperature and supercooled temperature regionsfrom one another; provide mechanical supportfor the superconductingfield winding and thermal shield, and transmit the steady state machinetorque and may bemade of austinitic stainless steel or other materialwith low thermal conductivity, high strength and high modulus ofelasticity. Surrounding inner thermal shield 44 is a second vacuumcompartment 50. Surrounding superconducting field winding 26 and fixedto rotor 12 is outer shield 52 which includes two layers; one layer 54performs primarily the mechanical-thermal function and may be integrallyformed with shaft 24. The other layer 56 is mounted on layer 54 andfunctions primarily as an electrical shield. Layer 56, the electricalshield, extends into gap 58 between stator 14 and rotor 12.

The construction of outer shield 52 is shown in more detail in FIG. 2where like parts have been given like numbers. The electrical shield,layer 56, is typically formed of phosphor bronze, beryllium, copper orother copper alloys and is located on the side of shield 52 towards gap58 and armature 16 and away from superconducting field winding 26. Themechanical-thermal layer 54 is typically formed of stainless steel andis on the side of outer shield 52 facing toward field winding 26. Bothlayers 54 and 56 are at ambient temperature as is gap 58 and armature16. Inner thermal shield 44 is typically approximately 20 K and fieldwinding 26 is typically at 4.2 K as when it is being supercooled by asystem which uses liquid helium. A containment vessel 60 is used toenclose field winding 26.

Typical dimensions of a shield for a one thousand MVA superconductinggenerator are as follows: armature 16 has an inner radius of 19.5 inchesand an outer radius of 25.0 inches and is approximately 121 inches inlength; laminated iron magnetic shield 20 has an inner radius of 29inches and an outer radius of 41.7 inches; field winding support 28 hasan inner radius of 8.95 inches and an outer radius of inches;superconducting field winding 26 has an inner radius of 10 inches and anouter radius of 12 inches; containment vessel 60 has a radius of 12.0inches; inner thermal shield 44 has a radius of 12.1 inches; in outerelectricalthermal-mechanical shield 52 layer 54 has an inner radius of12.2 inches and an outer radius of 16.0 inches 'and layer 56 has aninner radius of 16.0 inches and an outer radius of 17.6 inches; theaxial length of shield 52 is at least co-extensive with that ofsuperconducting field winding 26 and may extend as far as or beyond eachend of armature 16 and extends circumferentially continuously aboutrotor 12. Further analysis of the problem in greater detail is containedin the thesis, cited supra, submitted in partial fulfillment of therequirements for the degree of Doctor of Philosophy at the MassachusettsInstitute of Technology by coinventor David Lee Luck, Electromechanicaland Thermal Effects of Faults Upon Superconducting Generators, June1971, available at the library of the Massachusetts Institute ofTechnology, a copy of which is submitted herewith for deposit in theU.S. Patent Office Scientific Library.

A shield according to this invention may be designed by determining itsmaterials of construction and its geometry. The field of choices isconstrained by the following considerations: it must have a given skindepth 8 for penetration of magnetic fields; it must have a given timeconstant for the decay of electrical currents within it; it mustattenuate time varying magnetic fields by a given amount; the lossesassociated with attenuation of externally applied time varying magneticfields should be small; it should withstand the combined mechanicalstresses due to electrical torques and centrifugal forces withoutdamage; it should withstand normal electrical forces without excessivedeflection; it should not be in mechanical resonance with the appliedtime varying mechanical forces; and it should have sufficient thermalcapacity to withstand electrical faults without overheating.Calculations justifying the shield design listed in Table 11-3 and shownin FIG. 11-7 of the Luck thesis follow.

The skin depth 6 is expressed:

where 8 the skin depth athe electrical conductivity of layer 56 or layer54 a,,= the magnetic permeability of layer 56 or layer 54 w,,= thefrequency of the applied magnetic field (assumed to vary harmonically)For example for a layer of phosphor bronze A Cu, 5% Sn) 0 9.35[(meter)/(Ohm) (millimeter) a 4'n-X10" [(Volt) (second)/(Amp) (meter)]typical values for 0), would be a), 378[1/(Sec)[ or m, 756 [l/(Sec)]corresponding values of 6 are:

8(378) 0.835 inches or 8(756) 0.59 inch for a layer 54 of stainlesssteel (18% Cr, 8% Ni) 0' 1.38 [meter/(ohm) (mil1imeter) 8(378) 2.17inches or 8(756) 1.54 inches If A 8 the layer is considered thick. If A8 the layer is considered thin. If A 8, exact formulations are required.In the thick and thin cases approximations may be used.

The time constant T, for a layer is expressed:

T k Ao'a R, (1+ A/2R,)

where i denotes inner radius of the layer and 0 denotes outer radius ofthe layer. The thickness A of layer 56 and of layer 54 may now be chosenas layer 56 A 1.6 inches and T 0.159 seconds and layer 54 A 3.8 inchesand T 0.0555 seconds and combined T 0.2145 seconds. The desirable value,from stability considerations for shield 52 for T is 0.212 seconds. Thisprovides optimum damping for a swing frequency of 1.61-12.

Fields at a frequency of m 756 (I/Sec) are frequently encountered(negative sequence fields). At these frequencies both the layer 56 andlayer 54 are considered thick.

layer 56 bronze A 1.6 inches 8 (756) 0.59 inch layer 54 stainless A 3.8inches 8 (756) 1.54 inches For these cases:

and

Ka 0.0066 for layers 56 Ka 0.0264 for layers 54 If 8 A the formulationfor Ka becomes:

The losses in the layers should be kept small. For a thick layer thelosses are:

where H is the external field with no shield present, and n is thenumber of pole pairs in the machine. The expressions for the varioustorques developed are quite complex and are set forth in detail in theLuck thesis, see specifically expressions A-33 through A-37. For the1,000 mva machine described in the Luck thesis where n I and H 0.IHproducing power then p 141,000 watts. The formulation is more complex inthe case of thin layers.

Fault torques are given by the expressions A-33 through A-37 in the Luckthesis. These forces plus the centrifugal stress given by pm R areapplied simultaneously to the layers. They must be combined 'to find theprincipal stresses. This is done by using a geometrical constructionknown as Mohres Circle. These forces, torques and stresses are tabulatedat page 58 of the Luck thesis.

The deflection of the layers can be calculated by use of equation A"-66in the Luck thesis and is tabulated on page 58 of that thesis.

The natural frequencies of the layers are given by equation A-67 in theLuck thesis and are tabulated on page 58 of that thesis.

The thermal capacity is given by Av mcAT where Av change in internal,energy of the layers m mass of the layers c specific heat of the layersAT= change in temperature of the layers for bronze c 0.09 (Btu/1b F) forstainless c 0.12 (Btu/lb F) The energy input to the layers is calculatedby knowing the value of the unbalance current 1,. The energy input isproportional to [#2. Values of If! corresponding to varying values of ATare given on page 59 of the Luck thesis.

The field of this invention is not limited to use in machines in whichthe superconducting field winding is rotating nor in machines in whichthe superconducting field winding is in the center and the armaturesurrounds it. For example, in FIG. 3 where like parts have been shownwith like numbers primed and double primed with respect to FIGS. 1 and2, a cryogenic electrical machine is shown in which rotor 12' is stillcentrally located within and surrounded by stator 14' but in machinerotor 12' includes simply shaft 24' with armature 16' rather than thefield winding carried on it as pictured in FIGS. 1 and 2, Thus thearmature structure in machine 10 includes simply part 18' which isformed integral with shaft 24'. In contrast stator 14' includes fieldwinding structure 25' which includes iron magnetic shield 20 and supportframe 70 in which is disposed superconducting field winding 26' mountedon field winding support 28' interconnected by means of thermal distancepieces 30' and '32 and 34' and 36' with the end pieces 72 and 74 ofsupport frame 70. There are two inner thermal shields 44 and 44"surrounding superconducting field winding 26. Inner thermal shield 44'supported on end pieces 46', 48 is separated from superconducting fieldwinding 26 by vacuum 42' and from frame 70 by vacuum 50. The secondinner thermal shield 44" is mounted on end pieces 46" and 48" and isseparated from field winding support 28' by vacuum 42" and from frame 70by vacuum 50". The outer electrical-thermal-mechanical shield 52' ismounted on frame 70 of stator 14' with its thermal-mechanicallayer 54'illustrated as formed integrally with frame 70 and its electrical layer56 mounted externally thereon facing gap 58' and armature 16 on internalrotor 12. It can be seen by comparing FIGS. 1 and 3 that outer shield 52is mounted with the superconducting field winding and with itsthermal-mechanical layer 54 facing toward the superconducting fieldwinding and its electrical layer 56' facing toward the armature. If inFIGS. 1 and 3 rotor 12' was held stationary and stator 14' was made tobe rotated so as to become the rotor the position and orientation ofshield 52' and the layers would be the same.

It may be necessary to increase the thermal capacity of outer shield 52without also increasing the electrical time constant. One way ofaccomplishing this result is to provide, FIG. 4, circumferential slots80 in layer 5 6 which functions primarily as an electrical shield. The

. use of slots 80 enables the thickness of the shield to be increasedand to thus increase the thermal capacity of the shield within a thermalskin depth and yet the slots keep the longitudinal electrical resistanceof the shield from simultaneously decreasing and thereby keeps theelectrical time constant from increasing.

Other embodiments will occur to those skilled in the art and are withinthe following claims.

What is claimed is:

1. In a cryogenic electrical machine having a superconducting fieldwinding mounted on a field structure and an armature vmounted on anarmature structure carried one by the rotor the other by the stator ofthe machine and an inner thermal shield surrounding, mounted with, andspaced from the superconducting field winding anelectrical-thermal-mechanical outer shield mounted with and spaced fromsaid superconducting field winding and located between saidsuperconducting field winding and said armature, said outer shield beingat least co-extensive axially with said superconducting field windingand circumferentially continuous between said superconducting fieldwinding and said armature and disposed in an ambient temperature regionof said machine, said outer shield including circumferential slots onone of its sides to provide significantly increased thermal capacitywithout significantly increased axial electrical conductivity.

2. In a cryogenic electrical machine having a superconducting fieldwinding mounted on a field structure and an armature mounted on anarmature structure carried one by the rotor the other by the stator ofthe machine and an inner thermal shield surrounding, mounted with, andspaced from the superconducting field winding anelectrical-themal-mechanical outer shield mounted with and spaced fromsaid superconducting field winding and located between said super- 8 afirst layer and a second layer said first layer Operating primarily asan electrical shield and said second layer primarily as a thermal andmechanical shield and said slots are disposed in said first layer.

ll =0 l

1. In a cryogenic electrical machine having a superconducting fieldwinding mounted on a field structure and an armature mounted on anarmature structure carried one by the rotor the other by the stator ofthe machine and an inner thermal shield surrounding, mounted with, andspaced from the superconducting field winding anelectrical-thermal-mechanical outer shield mounted with and spaced fromsaid superconducting field winding and located between saidsuperconducting field winding and said armature, said outer shield beingat least co-extensive axially with said superconducting field windingand circumferentially continuous between said superconducting fieldwinding and said armature and disposed in an ambient temperature regionof said machine, said outer shield including circumferential slots onone of its sides to provide significantly increased thermal capacitywithout significantly increased axial electrical conductivity.
 2. In acryogenic electrical machine having a superconducting field windingmounted on a field structure and an armature mounted on an armaturestructure carried one by the rotor the other by the stator of themachine and an inner thermal shield surrounding, mounted with, andspaced from the superconducting field winding anelectrical-thermal-mechanical outer shield mounted with and spaced fromsaid superconducting field winding and located between saidsuperconducting field winding and said armature, said outer shield beingat least co-extensive axially with said superconducting field windingand circumferentially continuous between said superconducting fieldwinding and said armature and disposed in an ambient temperature regionof said machine, said outer shield including a first layer and a secondlayer said first layer operating primarily as an electrical shield andsaid second layer primarily as a thermal and mechanical shield and saidslots are disposed in said first layer.